Conventionally, booster devices, such as a negative pressure booster and a hydraulic pressure booster, are known as automobile brake mechanisms. As a booster device of this kind, an electric booster device utilizing an electric motor as a booster source is disclosed recently (see, for example, Patent Document 1).
The electric booster device disclosed in Patent Document 1 is structured as a single unit including a main piston reciprocated in accordance with an operation given to a brake pedal, a cylindrical booster piston fitted outwardly so as to be displaceable relatively to the main piston, and an electric motor which reciprocates the booster piston.
In this case, respective front end portions of the main piston and the booster piston as pistons of a master cylinder are in communication with a pressure chamber of the master cylinder. Brake fluid pressure is produced in the master cylinder by input thrust force applied from the brake pedal to the main piston, and by booster thrust force applied from the electric motor to the booster piston.